1. Field of the Invention
The present invention relates to a supercharger which includes a centrifugal compressor for compressing air with an impeller upon rotation thereof, a rotational shaft coupled to the impeller, a motor that comprises a stator mounted in a casing which houses the rotational shaft therein and a rotor mounted on the rotational shaft, and a journal bearing which holds the rotational shaft radially in position.
2. Description of the Related Art
Generally, centrifugal compressors are used as a supercharger for efficiently supplying compressed air. For example, they are used as an accessory for supplying compressed air to an engine or an accessory for supplying compressed air as an oxygen-containing gas to a fuel cell.
A centrifugal compressor comprises an annular stator and a rotor having a permanent magnet and disposed in the annular stator. The rotor rotates when windings on the stator are energized.
The rotor is mounted on a rotational shaft, which supports thereon an impeller for compressing air and a journal bearing that holds the rotational shaft radially in position. The impeller, the journal bearing, and the rotor which are mounted on the rotational shaft make up a rotary unit that is considerably heavy. In particular, since the components of the rotary unit are made of highly rigid materials to allow the rotor to rotate at a high speed, the rotary unit has a large weight as a whole.
When the heavy rotary unit is rotated at a high speed, its moment of rotary inertia is so large that it fails to perform an economic and efficient supercharging function.
There is known in the art a compressor disclosed in Japanese Laid-Open Patent Publication No. 09-509999 (PCT). The disclosed compressor comprises a rotatable shaft, a drive means for rotating the rotatable shaft, at least one impeller rotor stage mounted on the rotatable shaft, and a bearing means supporting the rotatable shaft. The rotatable shaft is substantially hollow in at least a portion thereof.
According to the above-mentioned Japanese Laid-Open Patent Publication No. 09-509999 (PCT), however, the rotatable shaft provides a heat transfer passage of reduced cross-sectional area because at least a portion of the rotatable shaft is hollow. Therefore, the amount of heat transferred from the rotor, or the journal bearing in particular, which generates much heat during rotation thereof, to the impeller rotor stage is reduced. The compressor thus fails to perform a good heat radiating function.